Field of the Invention
The invention relates to an image pickup apparatus, such as a digital camera, and more particularly to an exposure control technique for an image pickup apparatus having a live view function.
Description of the Related Art
Image pickup apparatuses, such as a digital camera, include one having a live view function. The live view function is a function that makes it possible to check an object image by sequentially outputting image signals continuously read out from an image pickup device and displaying images based on the image signals on an image display section, such as an LCD, provided e.g. on a rear surface of the image pickup apparatus.
Further, there has been proposed a technique in which, for focus checking purposes, a function of displaying a photographed image in an enlarged state on the image display section is added to the live view function (see Japanese Patent Laid-Open Publication No. H11-298791). According to the proposed technique, an image of a predetermined portion of the photographed image read out from the image pickup device is enlarged and displayed on the image display section. For enlarged display, there have been proposed a method of cutting out a partial area from a full-screen photographed image read out from the image pickup device, and a method of reading out only a partial area of a photographed image from the image pickup device in advance and displaying the image of the read area.
In the live view display, the reading frame rate is preferably a high-speed frame rate from the viewpoint of smoothness of a displayed image, and to realize the high-speed frame rate, a method is generally employed in which pixel signals for photographing a still image are read out by thinning out the same. Further, when a partial area is read out in advance for display, with a reduced thinning-out ratio setting compared with a case of the full-screen live view, image quality of the enlarged display becomes higher than when a partial area is cut out for display from the thinned-out image signals for the live view display, so that the display becomes more suitable for focus checking.
Further, there have also been proposed image pickup apparatuses having a plurality of live view display modes. For example, there has been proposed an image pickup apparatus which is capable of selecting between a mode in which live view exposure control is performed based on an exposure target value which is different from a value for use during still image photographing so as to always ensure a proper level of brightness of a live view display image, and a mode in which the live view exposure control is performed based on the same exposure target value as the value for use during still image photographing.
On the other hand, exposure modes of a camera include not only a program AE mode in which a shutter speed and an aperture value are automatically determined, but also a manual exposure mode in which one or both of the shutter speed and the aperture value is/are set by a user, a shutter speed priority mode, and an aperture priority mode. In these modes, there is a case where an image having a proper brightness cannot be photographed depending on values set by the user and object conditions.
To solve this problem, there has been proposed a technique for changing one or both of the exposure factors (shutter speed and aperture value), which were manually set, to a proper value or proper values with an accuracy with which manual setting can be performed, based on a result of photometry performed by a photometry unit (see Japanese Patent Laid-Open Publication No. H05-61097).
Further, there has been disclosed a setting shifting function for temporarily changing the shutter speed setting while a release button is being half-pressed, in a case where the shutter speed set in the shutter speed priority mode cannot achieve a correct exposure in a range within which the aperture value is set in a manner interlocked with the shutter speed. Further, there has also been disclosed a setting shifting function for temporarily changing the aperture setting while the release button is being half-pressed, in a case where the aperture value set in the aperture priority mode cannot achieve a correct exposure in a range within which the shutter speed is set in a manner interlocked with the aperture value.
The photometry during the live view display is performed using image signals read out from the image pickup device. More specifically, each analog image signal output from the image pickup device is converted to a digital image signal by an analog-to-digital converter, and then is used for photometry. A range of photometry luminance detected from image signals read out from the image pickup device at once is determined depending on dynamic range characteristics of the analog-to-digital converter, and for example, if the output of the analog-to-digital converter is 14-bit, the photometry luminance range corresponds to approximately ±three steps.
Here, photometry control performed during the live view display will be described with reference to FIG. 8. When measuring a luminance of an object having a brightness more than three steps beyond the brightness as a control target of the live view, it is necessary to perform photometric operation a plurality of times due to the above-mentioned restriction of the photometry luminance range.
For example, assuming that the shutter speed is set to 1/8000 sec in the shutter speed priority mode, and photographing is to be performed at an ISO value of 100, a lens having an open F number of 1.8 makes it possible to receive an appropriate amount of light from an object of which the correct exposure is achieved at an EV of 17. On the other hand, in a case where an object of which the correct exposure is achieved at an EV of 9 is to be photographed, the amount of received light becomes insufficient, resulting in an underexposed photograph. In this case, if the camera is in a mode for performing the exposure control of the live view display using the same exposure target value as that for the exposure control during still image photographing (hereinafter referred to as the exposure simulation mode), the object image is darkly displayed on the image display section.
The user viewing the image display section becomes aware that the exposure is not correct, and can photograph a correctly exposed image by changing the shutter speed setting and the ISO sensitivity setting. However, if the camera is equipped with the above-described setting shifting function, it is possible to photograph a correctly exposed image without having the settings changed by the user.
In the example illustrated in FIG. 8, the image display targeting a brightness corresponding to an EV of 17 in the exposure simulation mode is performed based on the shutter speed set by the user and the open F number of the lens in use. When the release button is half-pressed in this state, the setting shifting function operates, and by adjusting the photometry target value while performing photometric operation a plurality of times due to the above-mentioned restriction of the photometry luminance range, it is possible to calculate a final setting shift amount.
On the other hand, in a case where image signals of a partial area are read out from the image pickup device and used for enlarged display of the live view, image signals of the full screen cannot be acquired during the enlarged display, and hence such adjusting of the photometry target value as described above cannot be performed, so that it is impossible to calculate an accurate setting shift amount. In short, in a case where photographing is performed in the enlarged display state of live view, there is a possibility that an image photographed with correct exposure cannot be obtained.